Sphincter malfunction results from sluggish contractile response of the skeletal and smooth muscle of the sphincter. In the basal state, the smooth muscle of the sphincter remains in a state of tonic contraction and closure to serve as a one-way valve to regulate flow through the opening controlled by the sphincter. Skeletal muscle sphincters are under voluntary control while smooth muscle sphincters are controlled by complex interactions between extrinsic nerves from the central nervous system (CNS) and intrinsic control by the enteric nervous system and the myogenic properties of specialized smooth muscle cells.
Sphincteric smooth muscles represent tonic muscles that remain contracted at rest and have small amplitude, slow contraction and slow relaxation response, while non-sphincteric smooth muscle represent phasic muscle that shows a wide range of contractile activity varying from a fully relaxed basal state to a large-amplitude rapid contraction and rapid relaxation response (Goyal et al., The Gastrointestinal System, Motility and Circulation, in Handbook of Physiology, J. D. Wood and S. G. Schultz, Editors. 1989, The American Physiological Society: Bethesda. p. 865-908). It has been reported that tonic muscle may have lower levels of myosin light chain kinase and myosin light chain phosphatase than phasic muscle (Horowitz et al., Physiological Reviews, 76(4): p. 967-1003, 1996). The main contractile proteins are actin and myosin. The actin-binding proteins, such as tropomyosin, calponin and caldesmon, play a role in the thin filament based regulation of smooth muscle contractility (Reviewed in Chalovich, Pharmacology & Therapeutics, 55(2): 95-148, 1992).
Tissue culture was developed in the early 1900's as a technique for studying the behavior of animal cells in vitro. Advances in technology have led to the use of tissue culture for studying many areas of cellular function including intracellular activity, intracellular flux, environmental interaction, cell-cell interaction, and genetics. (Freshney, Culture of Animal Cells. New York: Wiley-Liss Inc, 1994) However, two-dimensional cell culture provides few options to study contractile force production as a cellular function. Furthermore, cells which are propagated and tested two-dimensionally are not exposed to cellular and environmental cues that may play a role in differentiation and the establishment of normal physiological function. Isolation of single cells and cell suspensions has enabled biochemical and mechanical measurements at the cellular level and has advanced the understanding of smooth muscle function. (Bitar et al., Am J Physiol. 260: G537-G542, 1991) Yet, cell suspensions also lack the potential for cell-cell interactions and cell-matrix interactions which are provided by three-dimensional tissues. (Freshney, Culture of Animal Cells. New York: Wiley-Liss Inc, 1994) It is well known that physiological functions of tissues are retained when the three-dimensional structure is kept intact. (Hungerford et al., Developmental Biology 178: 375-392, 1996) Isolated tissues and organ preparations, such as muscle strips, provide researchers with a three-dimensional tissue that may be subjected to controlled changes in perfusion, oxygen availability, and agonist-induced stimulation (Glavind et al., Am J Physiol. 265: G792-G798, 1993; Glavind et al., Am J Physiol. 272: G1075-G1082, 1997; Knudsen et al., Am J Physiol. 269: G232-G239, 1995). Despite the obvious advantages of tissue/organ explants, many limitations remain. Explanted tissues are composed of several different cell types, for example, muscle strips from the internal anal sphincter may be comprised of smooth muscle, and any combination of mucosal, epithelial, and/or neuronal cells. In addition, explanted tissues do not survive indefinitely and are usually only viable for up to four hours after isolation. This short-term viability prevents long-term investigation, and requires the explants be prepared de novo for each experiment. Damage to the tissue and/or release of material from damaged erythrocytes occurring during dissection also inhibits the ability to produce normal functionality and environmental conditions. Lastly, tissue/organ explants must be placed in a cold (usually 4° Celsius) buffer or bath to prevent rapid degradation.
Due to the limitations of investigating explant, tissue engineering has emerged as a valuable tool that applies the principles of engineering and life sciences toward the development of biological models with characteristics similar to those observed in vivo. Specific cell types can be isolated and bioengineered to yield a homogenous tissue. In addition, bioengineered tissues can be maintained in culture for long periods of time under physiological conditions. Advances in tissue engineering have been clinically applied to restore, maintain, and improve tissue function (Edelman, Circ Res. 85: 1115-1117, 1999; Valdivia, Curj 1: 6-11, 2001).